1. Field of the Invention
The invention relates to flat panel display technology, and in particular to an organic light-emitting diode (OLED) display comprising a thin film transistor (TFT) device having different electrical characteristics in a peripheral circuit region and an OLED driving region and a method for fabricating the same.
2. Description of the Related Art
The demand for active-matrix flat panel displays, such as active matrix organic light emitting device (AMOLED) displays, has increased rapidly in recent years. AMOLED displays typically employ thin film transistors (TFTs) as a switching element and a driving element as a light-emitting device in a pixel region. Additionally, AMOLED displays also employ a complementary metal oxide semiconductor (CMOS) circuit composed of TFTs in a peripheral circuit region.
Such elements are classified as amorphous silicon (a-Si) TFTs and polysilicon TFTs according to the active layer materials used. Compared with a-Si TFTs, polysilicon TFTs have the advantages of high carrier mobility, high driving-circuit integration and low leakage current, and are often applied to applications that require high-speed operation. Thus, low temperature polysilicon (LTPS) is a novel application for FPD technology. LTPS allows for an easier IC manufacturing process, which integrates driving circuits on a glass substrate having pixels thereon, reducing manufacturing costs.
During the LTPS-TFT fabrication, the TFTs in the peripheral circuit region and the pixel region have substantially the same electrical characteristics. In AMOLED displays, however, the electrical characteristics of the TFTs in the peripheral circuit region and the switching TFTs in the pixel region are different from the driving TFTs in the pixel region. For example, it is desirable to design the former with high carrier mobility and low sub-threshold swing, thereby providing a faster response. Additionally, it is desirable to design the latter with high sub-threshold swing and low threshold voltage to increase gray scale and extend OLED lifespan. However, it is difficult to fabricate TFTs with different electrical characteristics with the LTPS fabrication process.
Additionally, during the LTPS fabrication process, the active layers of TFTs are formed by a high power laser crystallization process. However, since the laser output energy is non-uniform, the driving current of each TFT for driving the OLED varies and thus, induces mura defects in displays.
Therefore, there exists a need in the art for development of an improved TFT device for OLED displays, having different electrical characteristics and capable of eliminating mura defects induced by driving TFTs of OLEDs.